The condition of the auditory nerve is thought to be one of the most important factors that predict the outcome of a cochlear implant (CI). The neural condition varies across the implanted ears and across cochlear regions within an ear. Speech recognition can potentially be improved if the auditory periphery is stimulated in a way that is th most optimal for each individual based on an assessment of the neural condition of the implanted ear across the electrode array. Assessing neural conditions in living human CI users has been possible by the establishment of non-invasive measures that correlate with the density of the spiral ganglion cells in the animal models. The objective of the current proposal is to assess these clinically-applicable measures of neural health for their applications in human CI users and design customized stimulation strategies based on these measures. In implanted guinea pigs, the count of the spiral ganglion cells near tested electrodes correlate with detection thresholds at low pulse rate, as well as the rate of threshold decrease with the increase of pulse rate, known as multipulse integration (MPI). Guinea pigs with lower thresholds and steeper MPI slopes tended to have higher spiral ganglion cell density. Aim 1 evaluates the two psychophysical measures to determine if they can be used to estimate neural density in human CI users. The prediction is that if the slope of the MPI functions, or the low- rate threshold, is dependent on neural density, the measure should predict place specificity of neural excitation (tuning) in humans. Aim 2 tests the hypothesis that the function of the surviving neurons is the underlying mechanism for the steepness of the MPI functions. The rationale for the hypothesis is that, in neomycin- deafened animals, in ears with low neural density, the surviving neurons are likely to also have impaired neural function, such as prolonged neural refractoriness and greater adaptation to stimulation. Both factors could result in a lower-than-expected neural sampling of the high-rate stimulus manifested as insensitivity to pulse rate change (i.e., shallow MPI slopes). The prediction is that if MPI slope is dependent on neural function, shallow MPI functions should predict slower neural recovery from refractoriness and adaptation to stimulation. Aim 3 proposes two customizing mapping strategies based on the proposed neural density and function measures. Strategy one targets stimulation at cochlear regions estimated to have high neural density by deactivating stimulation sites with spread of neural excitation. Strategy two lowers stimulation rate for subjects who show overall low sensitivity to pulse rate change. Speech recognition performance using these new strategies will be compared to that using the subjects' clinical maps. The proposed psychophysical measures for neural health and the customizing mapping strategies are simple and time-efficient and, thus have great potential for clinical use. 1